1. Field of the Invention
This invention relates to a device for automatically controlling the amount of a solid paint component in an electrodeposition coating tank by utilizing ultrasonic waves.
2. Description of the Prior Art
Electrodeposition coating involves passing an electric current through an electrically conductive article immersed in a tank containing an electrodeposition paint whereby the solid component in the electrodeposition paint is deposited on the surface of the article and forms a coated film. The solid component of the electrodeposition paint is thus carried away by the article and the amount remaining in the paint in the tank tends to decrease gradually. To achieve the desired film thickness and properties, therefore, the amount of the solid component of the electrodeposition paint should be maintained at a certain fixed value.
In the prior art, the amount of the solid component of an electrodeposition paint is measured by a drying residue method consisting of the following steps:
(1) measuring the weight (A grams) of a weighing dish,
(2) taking a paint sample into the weighing dish and measuring its weight (B grams),
(3) placing the weighing dish containing the paint in a hot air oven, heating it, for example, at a temperature of 105.degree. C. for 3 hours, evaporating water and the solvent, gradually cooling the sample in the hot air oven, taking out the sample when it attains room temperature, and measuring its weight (C grams), and
(4) calculating the proportion of the solid component of the paint in accordance with (C-A)/(B-A).
When the measured proportion of the solid component is lower than a fixed value beyond the tolerable range, the amount of a fresh replenishing paint of high solids content to be added is calculated from the balance between the measured proportion of the solid component and the desired fixed value proportion, and the fresh replenishing paint is sent to the electrodeposition coating tank by operating a pump.
The method of controlling the amount of the paint solid utilizing the drying residue method thus goes through steps of sampling, weighing, heating, gradual cooling and calculation, and requires a great deal of expense, time and labor. Furthermore, since this method does not permit real-time measurement, the amount of the solid paint component is liable to vary.
Another known method is to count the number of coated articles which have passed through the electrodeposition coating tank by a counter, and presuming the change of the amount of the solid component from (the amount of the paint coated per article).times.(the number of the coated articles). The change of the proportion of the solid component presumed by this method, however, has low reliability when the shapes and areas of the coated articles are not constant.